1. Field of the Invention
The present invention relates to a rewritable phase-change medium read-compatible with a compact disc, a recording method and a recording/retrieving apparatus for the medium. More particularly the invention concerns an improvement of overwriting performance at the 4-times or higher velocity.
2. Description of the Related Art
Generally compact discs (CD) are mediums to and from which a binary signal is recorded and a tracking signal is detected utilizing modulation of reflectivity resulting from interference of reflected light from the bottoms of phase-depth modulated pits and the mirror surface.
Recently, as a medium read-compatible with CD, a rewritable compact disc (CD-RW, CD-Rewritable) of phase-change type has been used (“CD-ROM Professional” published in the United States, September 1996, pp. 29–44 or Assembly of Manuscripts for Phase-Change Optical Recording Symposium, 1995, pp. 41–45).
The phase-change CD-RW detects a recorded information signal utilizing the change in reflectivity due to the refractivity difference between the crystal and amorphous states. The usual phase-change medium has a multi-layer structure including a lower protective layer, a phase-change recording layer, an upper protective layer and a reflective layer, which are disposed on a substrate one over another; utilizing multiple interference of these layers, the reflectivity difference and the phase difference are controlled to give a read-compatibility with CD. In CD-RW, the term “recording” means overwriting that is to erase existing data by writing new data, namely, to erase and record at the same time.
To this end, although the read-compatibility with CD inclusive of as high a reflectivity as 70% or more is difficult to achieve, the read-compatibility with CD can be secured in respect of the recorded signal and groove signal as long as the requirement for the reflectivity is allowed to be above 15% and below 25%. If an amplifier system for covering lowness of the reflectivity is added to a retrieving system, it is possible to realize retrieving by the current CD drive.
One of the common problems with CD-RW is slowness of the recording speed and data transfer rate.
The reference velocity (hereinafter also called the 1-times velocity) during recording/retrieving of CD is a linear velocity of from 1.2 to 1.4 m/s. For CD-ROM, high-speed retrieving of roughly 40-times velocity at maximum has already been realized; low-speed retrieving of 1-times velocity is limited to retrieving of music and image. Generally, for up to 16-times velocity, retrieving is made in a constant linear velocity (CLV) mode, which is the original mode of CD, and for 24- to 40-times velocities, retrieving is made in a constant angular velocity (CAV) mode to make a remarkable increase in data transfer rate and user data area and decrease seek time on the outer periphery of the recording area.
Attempts have been made to increase the recording speed in CD-RW. To this end, recording in CLV mode has become possible at only 1- through 4-times velocities. Usually, with CD-RW, it takes 74 minutes (or 63 minutes) to make recording throughout the entire recording area at the 1-times velocity, and it still takes 20 minutes to do so even at the 4-times velocity. But it takes only 10 minutes or less to record at the 8-times velocity or more, widening applications of CD-RW to recording of bulk data such as music and video.
External storage devices of the current computers are chiefly in the form of magneto-optical recording mediums (MO) whose data transfer rate is fast; if the data transfer rate of CD-RW could be increased, it is likely that such usage would be extended.
Consequently a phase-change medium with which faster recording is possible and a recording method for the medium have been cherished.
However, such CD-RW enabling recording at higher linear velocities has not yet been realized. It has been technically difficult to realize a medium that enables overwriting through a wide linear velocity range, whose low speed end is 1- and 2-times velocities and high speed end is 8- to 10-times velocities, without any risk of impairing the read-compatibility with existing CD-RW recording system overwritable at between 1- and 4-times velocity. This is true because the current medium and the recording method respectively encounter the following two problems:
The first problem is that it is difficult to resolve a trade-off in short-time erasure requirement by high-speed crystallization of amorphous marks and archival stability requirement of amorphous marks.
For example, AgInSbTe popular as material of the recording layer of CD-RW for recording at 1- through 4-times velocities could allow high-speed crystallization and hence 8-times-velocity recording if the content of Sb is relatively increased.
But according to studies of the present inventors, simply increasing the content of Sb would seriously impair the archival stability of amorphous marks so that the amorphous marks would disappear so as to be no longer retrievable within 1 or 2 years in room temperature and otherwise within several days in a high-temperature environment of 50–80° C. inside the recording apparatus. More serious, it turned out that amorphous marks would begin to disappear when retrieving has been repeated hundreds to thousands times by irradiation with laser beam with the power below 1 mW so that reliability for a recording medium could not be guaranteed.
Additionally, CD-RW has to be read-compatible in retrieving with the widely popular retrieving-dedicated CD-ROM drive. For example, for read-compatibility, it would be essential to satisfy high modulation in a range of 55–70% and reflectivity in a range of 15–25% as well as other servo signal characteristics.
The second problem is that according to CD-RW specifications, fairly strict recording pulse strategies (divided pulse method) are standardized. For example, recording at a wide range of linear velocities, from 4-times velocity to 8- through 10-times velocities must be carried out by the recording pulse strategy of FIG. 4, or a modified pulse strategy which is an analogy and does not require a considerable reconstruction of the current recording pulse strategy generation IC circuit, according to CD-RW specifications as normalized by Orange Book Part 3, Version 2.0.
In FIG. 1, (a) depicts the data signal having EFM-modulated time lengths of 3T through 11T, and (b) depicts the laser power of actual recording light generated based on the data signal. Pw is the recording power for forming amorphous marks by melting and then rapidly cooling the recording layer, and Pe is the erasure power for erasing by crystallizing amorphous marks. Usually, the bias power Pb is substantially the same as the retrieving power Pr of retrieving light.
When mark-length-modulated information is recorded on a phase-change medium in terms of different mark lengths, the ratio of the maximum to the minimum of the linear velocities in use is limited to nearly 2 times when the recording pulse strategy is strictly fixed.
Since many of the current CD-RW writers recordable at 4-times velocity can generate only a fixed waveform in accordance with the recording pulse strategy defined in the above-mentioned specifications, it was very difficult to realize high-speed recording at least at 8- through 10-times velocities without any risk of impairing the read-compatibility with these existing writers.